The present invention relates to a semiconductor laser device and a manufacturing method therefor and relates typically to a semiconductor laser device used suitably for an optical disk apparatus and an optical transmission module section of an optical transmission system and a manufacturing method therefor.
The present invention further relates to an optical disk apparatus and an optical transmission system provided with such a semiconductor laser device.
Semiconductor laser devices are widely used for optical disk apparatuses, optical transmission systems and the like. Among others, a semiconductor laser device called a buried ridge type has a high reliability and is known as a semiconductor laser device capable of operating with low power consumption (low threshold current). However, the buried ridge type semiconductor laser device must be manufactured through complicated processes requiring a second-time crystal growth process for forming a current constriction layer and a third-time crystal growth process for forming a contact layer in addition to a first-time crystal growth process carried out for forming a semiconductor layer that includes an active layer and a cladding layer in the manufacturing processes. The above therefore has led to the problems of degraded yield and high manufacturing cost.
Accordingly, there is a ridge waveguide type semiconductor laser device, which has a ridge portion on its active layer and is able to be manufactured through one-time crystal growth process (refer to JP H04-111375 A) as a conventional semiconductor laser device that can be manufactured more simply at lower cost.
FIG. 11 is a schematic sectional view of the conventional semiconductor laser device. The conventional semiconductor laser device is manufactured as follows.
First of all, an n-type InGaP cladding layer 402, an InGaAs/GaAs strained quantum well active layer 403, a p-type InGaP cladding layer 404 and a p-type InGaAs contact layer 405 are successively layered on an n-type GaAs substrate 401 by the MOCVD (metal-organic chemical vapor deposition) method. The p-type InGaP cladding layer 404 is etched partway by a photolithography technique or the like to form a mesa that becomes a ridge portion, and thereafter, Ti/Pt/Au and Au—Ge—Ni/Au are successively deposited as a p-electrode 406 and an n-electrode 407, respectively.
If a current is flowed through the device manufactured as described above, a Schottky junction portion 408 is formed between the p-type InGaP cladding layer 404 and the p-electrode 406, and a current flows only between the p-electrode 406 and the p-type InGaAs contact layer 405, effecting current constriction.
In contrast to the buried ridge type semiconductor laser device that requires the complicated manufacturing processes of three times of crystal growth processes in total as described above, the conventional ridge waveguide type semiconductor laser device is required to undergo only one-time crystal growth process. In addition, the conventional semiconductor laser device, which has a construction that achieves current constriction by using the Schottky junction different from the generally called air ridge type that employs an inorganic insulator for current constriction and belongs to the ridge waveguide type semiconductor laser devices, has thus a simpler structure and is manufacturable at lower cost.
However, it has been discovered that the conventional semiconductor laser device of JP H04-111375 A has had the following problems. That is, differently from the buried ridge type semiconductor laser device and the air ridge type semiconductor laser device described above, the conventional semiconductor laser device described the above document has an electrode put in direct contact with the side surface of the ridge portion and the surface of the cladding layer that extends sidewise from the ridge portion. In this case, depending on the refractive index of the semiconductor material that constitutes the semiconductor laser device and the refractive index of the metal material that constitutes the electrode, the distribution of the oscillation laser light has sometimes became easy to leak toward the electrode formed on the side surface of the ridge portion and the surface of the cladding layer located in the neighborhood of the ridge portion.
Regarding the refractive index of the electrode material located on the ridge side employed in the conventional semiconductor laser device described in JP H04-111375 A, the material of Ti put in contact with the semiconductor layer has a refractive index of about 3.0 to 3.6 within a wavelength range of 650 nm to 1.5 μm, and the material of Pt provided on Ti has a refractive index of about 2.9 to 5.5 within the same wavelength range of 650 nm to 1.5 μm. On the other hand, the effective refractive index in a direction perpendicular to the substrate outwardly of the ridge portion is also, for example, about 3.2, and therefore, the refractive indexes of Ti and Pt become unignorable.
As described above, if the effective refractive index in the perpendicular direction outwardly of the ridge portion is close to the refractive index of the electrode material formed directly on the semiconductor layer, the oscillation laser light sometimes becomes easy to leak toward the electrode side.
The buried ridge type semiconductor laser device described above, in which the buried layer that is made of a semiconductor material and provided for current constriction exists on the p-type cladding layer excluding the ridge portion, and the semiconductor layer that becomes a contact layer is formed on the ridge portion and the p-type cladding layer, has had no such problems. Moreover, the air ridge type semiconductor laser device, in which the electrode is formed on the inorganic insulator provided for current constriction on the side surface of the ridge portion and the surface of the cladding layer that extends outwardly of the ridge portion, has not been required to take such a problem into consideration.
However, light sometimes leaks toward the electrode side formed on the ridge side in the conventional semiconductor laser device described in JP H04-111375 described above. If such a phenomenon occurs, since the metal material that constitutes the electrode generally has an optical absorption coefficient being about 104 to 105 times higher than that of the semiconductor material, the metal material that constitutes the electrode becomes a very large light-absorbing component, disadvantageously largely increasing the internal loss. It has consequently been discovered that the conventional semiconductor laser device has disadvantageously caused the problems of a reduction in the slope efficiency or an increase in the oscillation threshold current value.